1. Field Of The Invention
The present invention relates generally to a method and apparatus for calibrating resistance bridge-type transducers, and more particularly, to such a method and apparatus whereby a source of substantially constant current is selectively switched across one of the resistance elements of the bridge for calibration purposes.
2. Brief Description of The Prior Art
Bridge-type transducers have traditionally been checked by noting the change in transducer output voltage caused by switching a known-resistor in parallel with one of the arms of the bridge. This procedure is referred to as "shunt calibration".
This term is actually a misnomer insomuch as the procedure does not actually determine the sensitivity of the bridge to a change in the measurand. Nevertheless, shunt calibration is a very useful test since it checks the integrity of the transducer, cabling, and signal-conditioning equipment. Indeed, it is hard to imagine any system fault short of the transducer falling off that would not be detected by shunt calibration.
The principle drawback of shunt calibration, as it is currently practiced, is that a given shunt resistor produces a different output from every transducer since no two transducers have exactly the same resistances. Furthermore, the arms of silicon PR transducers have large temperature coefficients of resistance, so the output produced by a given shunt resistor changes with temperature.
In principle, these drawbacks could be overcome by building the shunt calibration resistor into the transducer. The calibration resistor could be trimmed to produce the desired offset, and could be made with the same temperature coefficient as the bridge resistors. The calibration resistor could be switched in and out remotely by an electronic switch. Practically, it is most difficult to trim a diffused silicon resistor, and the electronic switch would require low, stable "on" resistance and high "off" resistance.
Other disadvantages and complexities of shunt calibration will be discussed hereinafter.